An aircraft pilot utilizes instrumentation to assist in the understanding of flight situations at any instant. Instrumentation systems are available and provide compact integrated displays which portray the horizontal situation of an aircraft in any one of several available modes. It is also desirable to generate autopilot heading command signals in horizontal situation indicators of the type just described for aircraft control. It is further desirable to incorporate radio-generated horizontal situation indication when radio navigation in either the localizer mode or omni mode is selected.
An instrument display of the type to which the present invention is directed indicates several distinct kinds of information to the pilot, each of which will be described now in more detail.
A heading card is provided to indicate aircraft heading. The card is in the form of a flat ring suitably scaled in a compass format on the face thereof. Provision is made for synchronous rotation of the heading card with a directional gyro. The heading card rotates with respect to a fixed lubber line to indicate the present compass heading of the aircraft.
The heading card cooperates with a heading marker to provide for selection of the desired aircraft heading in the heading mode. The heading marker is movable relative to the card for selecting a desired autopilot command heading.
Centered in the heading card is a radio display which provides an indication of the horizontal situation generated from a radio navigation instrument. A meter movement is responsive to deviation from the radio path and thus indicates such displacement to the pilot. Movement of the meter needle across the face of the radio display indicates the extent of the displacement from the radio path. The meter needle forms the center portion of an arrow-shaped indicator (omni needle) which extends across the face of the heading card. Like the heading marker, the omni needle is movable relative to the card for selecting a desired omni bearing.
Directional gyros of the type used in aircraft instrumentation are sensitive to torques applied from external sources. Such torques must be minimized in order to avoid "spilling" the gyro. A directional gyro is spilled when a torque applied to the outer gimbal causes precession of the inner gimbal until it hits a stop. The outer gimbal then moves in an uncontrolled manner and the instrument ceases to function as a gyro.
A major source of external torque in prior art systems has been the means employed to generate an indication of the relative positionings between the heading card and heading marker (center section) or omni needle. In many prior art systems mechanical differentials have been employed to generate these signals. Such mechanical differentials have one shaft connected to the heading card and a shaft connected to the center section. When there is relative motion between the heading card and the center section an output develops from the differential. All such mechanical differentials apply some torque to the heading card when in operation.
Prior art systems have used various means for coupling the directional gyro to the heading card and isolating the gyro from the torques described above. For example, a gyro, remotely located from the instrument panel, generates an electrical signal that is applied to a servo indicator driving a heading card.
Another technique has been to mechanically isolate the gyro from the heading card system and provide a feedback-controlled servo system to couple the heading card and the gyro. A tracking disk is rotatably mounted on the vertical outer gimbal axis of the gyro with a pickoff for detecting the angular displacement between the tracking disk and the outer gimbal. The pickoff and tracking disk are configured such that relative rotation may occur with relatively low friction. A transmission means couples the disk and the heading card for equal rotation, and a servomotor drive for the transmission means slaves the disk to the gyro in response to an error signal from the pickoff. Such a disk system is disclosed in U.S. Pat. No. 3,849,896 entitled "Directional Gyro Indicator System."
In the system just described, the servo and transmission means which rotate the card can withstand torques applied thereto by the mechanical differentials. The pickoff and tracking disk system generates an electrical signal to operate the servo, and thus external torques are not applied directly to the gyro.
Although a system such as the one described above is the best device currently available, it is less than ideal in several respects. The transmission system and the tracking disk/pickoff system are relatively complex and difficult to arrange in the required small package. This complexity increases the cost of manufacture, because the system requires careful alignment of many cooperating parts. The complexity and associated high cost is directly caused by the need to provide mechanical isolation of the gyro from mechanical torques.
Thus, it can be seen that a need has arisen for a horizontal situation indicator which is simple in operation and construction yet which allows isolation of the directional gyro from external torques.